1. Field of the Invention
This invention relates to a process for purifying flue gas generated during the combustion of waste materials, such as municipal refuse, and containing gaseous, liquid, and particulate pollutants, and more particularly to flue gas purification processes in which soluble heavy metals, including mercury, are precipitated, the mercury recovered in a thermal treatment, and the solid residues returned to the combustion chamber to pyrolyze noxious organic substances and/or combined with a hazardous waste stabilizing substance.
2. Discussion of the Art
In flue gas purification systems, solid, liquid, and gaseous pollutants are separated from the flue gas with the aim of reducing damage to the environment to an acceptable degree. Particulate pollutants are typically separated from the flue gas leaving a combustion system by mechanical or electrical dry separation means, such as fabric or electrostatic filters and cyclone equipment, respectively. Wet dust removal means utilizing absorption, adsorption, and chemosorption processes are frequently employed as well. Such wet dust removal processes also serve to remove liquid and gaseous pollutants from the flue gas by absorption, adsorption, condensation and chemical reaction processes, and produce sludges and waste waters having a high concentration of dissolved solids. These sludges and waste waters frequently contain potentially toxic or hazardous levels of waste materials, such as heavy metals and noxious organic substances and, thus, usually require further treatment in order to reduce the environmental hazard they would present if casually disposed of.
Flue gas purification systems generally combine several basic processes into one overall process. One of the most popular waste material combustion processes for treating, for example, municipal refuse, is discussed in Abfallwirtschaft an der Tu Berlin [Waste Treatment Policies at the Technical University of Berlin], Volume 7, pages 1-41. The process provides for the removal of particulates from flue gas by means of electrostatic filters or fabric filters and the partial removal of the gaseous pollutants HCl, HF, and SO.sub.2 from the flue gas by means of acid or alkali treatment, either upstream or downstream from the filter system. Heavy metals present in gaseous form, such as mercury vapor, and organic contaminants, such as chlorinated dioxins, however, have so far only been reliably removed by wet processes downstream from the filters.
A significant drawback of conventional processes is that they typically produce a large quantity of particulates, about 30 kg of smoke particulates per ton of municipal refuse burned. These particulates, moreover, contain high concentrations of halogens, noxious organic substances, and heavy metal substances, sometimes in a form easily transported in the ecosystem. For example, the composition of a typical flue gas from the combustion of municipal refuse is given in Table 1. Table 3 lists the dioxin content of a typical flue ash. Further, the mass balance for the most important heavy metals typically found in municipal refuse is illustrated in FIG. 1. As may be seen, the safe handling of these pollutants, especially the heavy metal mercury and the particularly noxious organic substances belonging to the family of dioxins, poses a significant problem, particularly with a view toward recovery and unobjectional reuse of smoke particulates or decontamination and long-term, environmentally safe storage. Further, municipal authorities are imposing progressively stricter safety regulations for the handling of such waste materials and for the storage thereof in special hazardous waste depositories.
In his previous U.S. patent application, Ser. No. 06/617,941, coinventor Hubert Vogg addressed the problem of removing heavy metal contaminants from the flue gas purification process in a compact and concentrated form. The majority of particulates were removed from the flue gas in a dry separation system and collected as a flue ash. The flue gas leaving the dry separation system was passed through a condensation/scrubbing system, where vapors, including water, HCl, and heavy metal vapors, were separated by condensation and any remaining particulates were scrubbed from the flue gas. An acidic condensate solution was produced in the condensation step and was used to extract soluble heavy metals from the flue ash and the soluble heavy metals were then subjected to a precipitation reaction to remove them from the process in a compact and concentrated form. These process steps produced about 1 kg of heavy metal-containing residue for each ton of refuse processed. Thus, heavy metals, such as mercury, cadmium, lead, antimony, tin, and zinc were compactly removed from the process and were made available for recovery and reuse. The soluble heavy metal-free residues from the referenced process were returned to the combustion system and were made inert by reacting them with the slag at high temperatures to become fixed therein. Further, the flue gas, previously rendered free of particulates, condensed vapors and heavy metals, was washed with an alkali wash solution to remove polluting gases, including SO.sub.2 and NO.sub.x, to provide a purified flue gas which may be discharged through a stack into the environment. The acid condensate solution, after precipitation of heavy metals therefrom, was then combined with the alkali wash solution in a neutralization step which produced a clarification sludge and a clarified solution. The clarified solution was dischargeable into a sewer, while the clarification sludge could be returned to the combustion chamber.
Even though such a process results in recovery of heavy metal pollutants in a more compact and concentrated form, noxious organic substances continue to pose a signficant problem in such a process, whether or not present initially in the municipal refuse, since extraction of soluble heavy metals from flue ash with an acidic condensate solution does not influence noxious organic substances. The very heterogeneous composition of the waste materials involved in a refuse burning process allows numerous chemical reactions to occur. On the one hand, many materials undergo decomposition or evaporation at the typically prescribed combustion temperature of 800.degree. C. On the other hand, reaction products released into the gas phase travel a long path from the combustion chamber through a heat exchanger and dry separtion means, while being continuously cooled down to about 200.degree. C., and recombination reactions may take place and new organic substances may form. Thus, aliphatic and aromatic chlorinated compounds may be formed by the above-described mechanisms.
Generally, such organic compounds are collected along with the flue ash. The filters of the dry separation system may thus contain organic compounds of widely varying compositions and toxicities and in widely varying concentrations. The presence of organic substances, such as chlorinated dibenzene dioxin or dibenzene furan, presents a particularly significant disposal problem in view of their toxicity even in trace quantities. Final decontamination of flue ash containing such toxic substances at a hazardous waste depository becomes more difficult and may be prevented entirely by local hazardous waste management ordinances.